Although sometimes listed as a triode output valve, the ML4 was the definitive descendent of the famous (or infamous) Type KL1 of 1926 and was primarily a professional and military type, little used in domestic radios. It was introduced in 1929 and an improved version appeared two years later. The original envelope shape would have been balloon.

It was quite widely used as an RF oscillator, as a single-valve phase splitter, and as a driver for larger valves.

The specimen shown is from the 1940s.

The classic envelope is 42 mm in diameter and, excluding the B5 base pins, is 93 mm tall.

References: Datasheet, ¹⁰⁴⁰ & ¹⁰⁴³. Type ML4 was first introduced^¶ in 1929.

 
Pin Connections

B5	1	2	3	4	5
	a	g1	h	h	k

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	Vg	mAa	ra	gm	Pout
	4.0	1.0	200	-13.5	17	3,700	2.7	0.65W

Thanks to Frank Philipse for supplying the above PDF datasheet

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